A lithium ion secondary battery is small in volume, has a large mass capacity density, and is capable of taking out a high voltage, and therefore, is widely adopted as a power source for small devices. The lithium ion secondary battery is used as, for example, a power source for mobile devices such as a cellular phone and a notebook-sized personal computer. In addition to its application to small devices, in recent years, due to concern for environmental issues and increasing awareness of energy-conservation, an application of the lithium ion secondary battery to large secondary batteries has been expected in the field where a large capacity with long life is required such as electric vehicles (EV) or electric power storage.
An electrode of a secondary battery is formed by forming an electrode mixture layer on a collector, and the electrode mixture layer is consisting of an active material, a conductive assistant, a binder, and so on. The binder has a function of adhering the active materials each other, and adhering between the active material and the collector. In view of battery performance, and easiness of compatibility with battery production process and so on, it is desired that the binder is excellent in basic properties such as electrochemical stability, resistance to electrolyte solutions, adhesiveness, and heat resistance. On the other hand, in view of the recent demand for cost reduction of large batteries, it is desired that the materials are as inexpensive as possible.
In negative electrodes of the lithium ion secondary batteries, aqueous binders using a rubber type latex such as styrene butadiene rubber (SBR) and a thickener such as CMC (carboxymethyl cellulose) in combination have often been used in addition to solvent-based binders such as polyvinylidene fluoride (PVDF). On the other hand, in positive electrodes, binders other than PVDF or fluoropolymers having a composition close to that of PVDF have hardly been put into practical use.
PVDF has high performance in various properties such as oxidation resistance, heat resistance, adhesiveness and resistance to electrolyte solutions, and is excellent in balance among these properties. Moreover, when PVDF is used, it is easy to obtain electrode slurry having a good coating property. However, there have been problems that the resin price of PVDF is as high as around 2000 yen/kg which is higher than that of other resins, and PVDF has a drawback in terms of alkali resistance. On the other hand, there has yet been found no material that can substitute for PVDF in terms of properties, and PVDF has still been used for long years.
As a binder other than PVDF, for example, Patent Literature 1 discloses that polyvinyl chloride (PVC) is used as a binder containing a halogen element as PVDF. Patent Literature 2 to Patent Literature 5 also disclose polyvinyl chloride as an example of the binder. Polyvinyl chloride (PVC) is a general-purpose resin and is very inexpensive.